Modem industrial society employs a multiplicity of devices which produce electromagnetic fields. To protect the health of humans, national and international standards and laws stipulate maximum limit values for such fields which must not be exceeded. In this context, the basic measured variable used to quantify electromagnetic exposure in a high-frequency field is the specific absorption rate (SAR). It indicates the energy per mass unit absorbed in the tissue. In the case of near-field exposure, such as the kind that occurs, for example, when portable cellular telephones are used, the SAR limit values relate to the maximum absorption occurring in the human body, averaged over 1 g or 10 g of tissue mass. The SAR distribution in a person can be determined by numerical simulation calculations or by measurements made using electrical field probes in phantom models. The main advantage one derives from measurements, as compared to calculations, is the ability to simply and reliably ascertain differences and variances between individual devices that produce electromagnetic fields.
Since it is not possible to measure SAR values directly in a living organism, phantom models of the human body or of body parts, such as the head, are used, which reflect real conditions fairly well. A measuring station frequently used nowadays to determine the SAR values produced in the human head by portable cellular telephones is described by Q. Balzano, O. Garay and T. J. Manning in xe2x80x9cElectromagnetic Energy Exposure of Simulated Users of Portable Cellular Telephonesxe2x80x9d, IEEE Transactions on Vehicular Technology, vol. 44, no. 3, August 1995, pp. 390-403. It describes externally attaching the portable cellular telephone being tested to a phantom model simulating the form of the human head. The phantom model is made of a thin plastic shell and is filled with a liquid that simulates tissue. An electrical field probe is moved within this liquid with the aid of a computer-controlled industrial robot. In the process, the computer records the electrical field strength values or SAR values over a three-dimensional grid network in the phantom model and, from the individual values, calculates the SAR values, averaged over 1 g or 10 g tissue mass, which are relevant to limit values. Apart from the undoubted advantages associated with a measuring station of this kind, there are various drawbacks with respect to systematic serial tests and development tests:
a) Substantial investment and operational costs are entailed in using an industrial robot including three-dimensional control.
b) The time needed to complete one measuring run at one single position of the portable cellular telephone is 30 minutes or longer. It usually takes several measuring runs to determine the least favorable position having the highest SAR value.
c) Moving the probe inside the phantom requires filling the phantom with a liquid. Since, over time, the liquid evaporates and its electrical properties change, one must constantly monitor the parameters of the liquid.
It is an object of the present invention to overcome the disadvantages of the known measuring station in qualifying devices which produce electromagnetic high-frequency fields.
In the measuring apparatus according to the present invention, the electrical field probe is not moved inside the phantom model, but is in a defined, fixed position, preferably 5 to 9 mm below the surface. This eliminates the need for mechanical movement, or for control and evaluation thereof. Instead, the device to be tested (e.g., a portable cellular telephone) is moved by hand or using a simple moving mechanism, past the outside surface of the phantom, it also being possible to determine the influence of the hand.
Simple mechanical auxiliary devices made of non-conducting plastic can be used to set the positions (e.g., those specified in a standard) of the device being tested. Through specific trials, one can establish the position in which the indicated specific absorption rate is at its maximum, with little outlay and within an extremely short period of time.
The present invention employs a phantom model of a solid material made, for example, of a combination of synthetic resin, ceramic powder and graphite powder. T. Kobayashi et al. reveal in xe2x80x9cDry Phantom Composed of Ceramics and Its Application to SAR Estimationxe2x80x9d, IEEE Transactions on Microwave Theory and Techniques, vol. 41, no. 1, January 1993, pp. 136-140 that solid materials are also suited for simulating the electrical properties of human tissues. As will be explained in the following, precise compliance with the tissue parameters is only of minor importance with regard to the measuring accuracy of the apparatus according to the present invention. The solid phantom model is non-toxic, simple to use, and does not change its properties over time.
Since the probe is in a fixed location in the loss-encumbered phantom, the outlay required to derive the detected signal is considerably less than that entailed when using highly complex probes, as required in liquid phantoms. The probe signal merely needs to be amplified using a simple electronic circuit and fed to a display instrument which produces the time-averaged value.